Tear proteins include lysozymes, lactoferrin and albumin. Once deposited on a contact lens, these proteins are subject to denaturation—which can incite an immunological response. Lysozyme accounts for 90% of the total lens protein deposits and has been detected on lenses after just one minute of wear.1 While this protein deposition is influenced by tear composition, it is modified by the chemical c haracteristics of the lens material, such as water content, pore size, roughness of the surface, hydrophobicity and ionicity, as well as protein factors, such as protein size, charge and length of material exposure.1-3
Contact lens solutions may have a modulating effect on protein deposition and denaturation as well. In fact, some large molecular proteins, like albumin, actually aid in the wetting of the lens surface.4 Many patients report decreased awareness of the lens on the second day, presumably due to biofilm deposition. Small proteins, such as the positively charged lysozyme, penetrate readily into negatively charged materials.
In their native (or active) state, lysozymes are bacteriolytic enzymes that defend the eye against pathogens. The adsorption of protein is only a problem when the protein denatures. High levels of lysozyme deposition have been measured in the matrix of group IV materials. But, silicone hydrogel materials have shown reduced protein deposition—although they have a greater percentage of denatured lysozyme, albeit very small amounts.5,6
Denatured protein has been implicated in contact lens complications such as giant papillary conjunctivitis (GPC) and inflammation, both of which are associated with dry eye symptoms and decreased contact lens comfort.7 Protein denaturation is a complex process that is known to be influenced by contact time with and chemical composition of the contact lens material, surrounding pH, type of protein and temperature.8 There are extreme variances between patients in the amount of protein that denatures. If a contact lens solution could remove lysozyme deposits and/or prevent protein denaturation, there is potential for a reduced immunological response.
Multi-purpose solutions, such as newly introduced Biotrue (Bausch + Lomb), Opti-free Replenish (Alcon), Complete EasyRub (Abbott Medical Optics) and RevitaLens Ocutec (Abbott Medical Optics) contain agents that specifically affect protein deposition and denaturation. The agents used in contact lens solutions include surface active agents which emulsify and remove dirt, mucus and lipid debris and ionic displacement cleaners (which pull positively charged deposits like protein from the negatively charged lens surface). Ionic surfactants disassociate into ions and can be categorized as one of three types: cationic (positive charge), anionic (negative charge) and amphoteric (either positive or negative).
The molecule sulfobetaine in Biotrue is a zwitterion, which surrounds and aids to protect lysozyme, so it remains in the native state and retains its inherent activity. A zwitterion is a molecule with both positive and negative charges, but with a net charge of zero. Additional Biotrue surfactants, including the block copolymer tetronic 1107, are attracted to denatured protein on the lens surface, helping to effectively lift and dissolve them.
Opti-free Replenish uses negatively charged Citrate to pull the positively charged proteins from the lens. Citrate has a negative charge that is ten-times greater than the negative charge of the contact lens. The surface active agent in Opti-free Replenish has the proprietary name TearGlyde, which is a combination of Tetronic 1304 + C-9 ED3A. Complete uses the surfactant Poloxamer 237.
Fortunately, in today’s contact lens market, lysozymes play a very small role in patient comfort. The truth is—except in group IV lenses, very little protein is deposited on the lenses and is probably more patient-dependent than anything else.
1. Senchyna M, Jones L, Louie D, et al. Quantitative and conformational characterization of lysozyme deposited on balafilcon and etafilcon contact lens materials. Curr Eye Res. 2004 Jan;28(1):25-36.
2. Lord MS, Stenzel MH, Simmons A, Milthorpe BK. The effect of charged groups on protein interactions with poly(HEMA) hydrogels. Biomaterials. 2006 Feb;27(4):567-75.
3. Garrett Q, Laycock B, Garrett R. Hydrogel lens monomer constituents modulate protein sorption. Invest Ophthalmol Vis Sci. 2000 Jun;41(7):1687-95..
4. Luensmann D, Jones L. Albumin adsorption to contact lens materials: a review.Cont Lens Anterior Eye. 2008 Aug;31(4):179-87.
5. Carney FP, Morris CA, Milthorpe B, et al. In vitro adsorption of tear proteins to hydroxyethyl methacrylate-based contact lens materials. Eye Contact Lens. 2009 Nov;35(6):320-8.
6. Suwala M, Glasier MA, Subbaraman LN, Jones L. Quantity and conformation of lysozyme deposited on conventional and silicone hydrogel contact lens materials using an in vitro model. Eye Contact Lens. 2007 May;33(3):138-43.
7. Skotnitsky C, Sankaridurg PR, Sweeney DF, Holden BA. General and local contact lens induced papillary conjunctivitis (CLPC). Clin Exp Optom. 2002 May;85(3):193-7.
8. Subbaraman LN, Jones L. Kinetics of lysozyme activity recovered from conventional and silicone hydrogel contact lens materials. J Biomater Sci Polym Ed. 2010;21(3):343-58.